This invention relates to electrophotographic marking systems that incorporate raster scanners having laser diode light sources. More specifically, it relates to such systems which compensate for the turn-on and turn-off times of their laser diode light sources.
Electrophotographic marking is a well known method of copying or printing documents or other substrates. Electrophotographic marking is performed by projecting a light image representation of a desired final image onto a substantially uniformly charged photoreceptor. That light image then discharges the photoreceptor so as to create an electrostatic latent image of the desired image on the photoreceptor""s surface. Toner particles are then deposited onto that latent image, forming a toner image on the photoreceptor""s surface. That toner image is subsequently transferred, either directly or after an intermediate transfer step, and fused onto a marking substrate such as a sheet of paper, thereby forming the desired image. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the creation of another image.
While many types of light exposure systems have been developed, a commonly used system is the raster output scanner (ROS). A raster output scanner is comprised of a laser beam source, a modulator for modulating the laser beam (which, as in the case of a laser diode, may be comprised of the source itself) such that the laser beam contains the desired image information, a rotating polygon having at least one reflective surface, input optics for collimating the laser beam, and output optics for focusing the laser beam into a spot on the photoreceptor and for correcting various optical problems such as wobble. The laser source, modulator, and input optics produce a collimated laser beam that is directed onto the reflecting surface or surfaces of the polygon. As the polygon rotates the reflective surface causes the laser beam to sweep along a scan plane. The swept laser beam passes through the output optics and is reflected by the mirror(s) so as to produce a spot which sweeps in a scan line across a charged photoreceptor. Since the charged photoreceptor moves in a direction which is substantially perpendicular to the direction of the sweeping spot, the sweeping spot raster scans the photoreceptor.
To assist the understanding of the present invention several things should be further described and highlighted. First, an electrophotographic printing machine may be required to image millions of individual spots on a given page. For example, a 300 spot per inch printer that images an area of 7.5 inches by 9 inches images an area containing more than 6 million spots. Considering that higher resolutions, such as 400, 600, 800 or 1200 spots per inch, are becoming common place, and considering that color printing requires that each image are be imaged 4 times (one for each of three primary colors plus black), a final print might be comprised of more than 100 million potentially imaged spots. Second, modern electrophotographic machines are frequently high volume machines in which the time available to print a desired image is severely limited. The foregoing implies that the time available to image each spot is very short.
After great effort and expense, manufacturers of electrophotographic printing machines have developed laser light sources and photoreceptor materials that generally meet the resolution and speed requirements of modem electrophotographic machines. However, as researchers attempt to improve the quality of the printed image still further, it has become apparent that improving the image quality of the leading and trailing edges of solid sections of an image area, such as the edges of a solid vertical line, is difficult. This is referred to herein as the edge placement problem.
One cause of the edge placement problem relates to the finite response times of laser diodes. Simply put, laser diodes take time to turn on when drive power is applied and they take time to turn off when drive power is removed. For example, after application of drive power to a laser diode it might take 1-5 ns for the laser diode to begin emitting light and another 0.1 to 10 ns for the laser intensity to reach its maximum. After drive power is removed it might take 0.5 to 15 ns for the laser diode to stop emitting light. Another cause of the edge placement problem is the finite response time of the laser diode drive signals. The overall effect of the laser diode and laser diode drive signal response times is a decrease in the accuracy of the edge placements.
Therefore, a technique of improving the edge placement by compensating for the finite response times of the laser diode and the laser diode drive signals would be beneficial.
The principles of the present invention provide for compensating for the finite response times of the laser diode and the laser diode drive signals. According to the principles of the present invention when an edge is being imaged the laser diode is turned on and off earlier than it normally would have been by advancing the occurrence of a controlling clock signal. Beneficially, the amount of advancement is such that an ideal instantaneous response time exposure curve crosses the actual laser response time curve at a place that preserves the placement of the developed line edge.
The principles of the present invention are beneficially implemented through incorporation of a data buffer, a processing capability that detects the starting edge and the ending edge of an image area, and a clock switch that advances the occurrence of a controlling clock signal over normal operation. When an image bit being produced is not a starting edge or an ending edge of an image area the image bit is produced in synchronization with a master clock single. When an image bit being produced is a starting edge or an ending edge the image bit is produced at an earlier time than it would have been if it was in synchronization with the master clock signal.